The degradation of elastin is a critical component of the pathogenesis of emphysema and the mechanisms by which macrophages participate in this degradation are completely understood. Prior work by the applicant, summarized in this application, has demonstrated that lung macrophages synthesize cathepsin L, a potent elastase, and that uptake of this enzyme from the extracellular microenvironment by a pathway with novel functional characteristics is a important determinant of intracellular enzyme levels. A strategy for the further characterization of this uptake pathway is presented, as well as proposed experiments to ascertain the role of this pathway in the intracellular accumulation of other proteases. The second major aim of this grant relates to the characterization and identification of a second elastolytic enzyme that has the characteristics of both a cysteine proteinase and a metalloenzyme. Data is presented demonstrating that one or more enzymes, with a MW of ~22 kD with an acidic pH optimum, is responsible for the majority of elastolytic activity seen in human lung macrophages. Further studies demonstrated that the activity of this enzyme(s) is increased 7 fold in smokers' macrophages. Experiments are proposed to further characterize the regulation of this enzyme, as well as to isolate and sequence it. Taken as a whole, these studies will allow the applicant to continue to explore the regulation of proteases in lung macrophages while developing the skills needed to become an independent investigator.